Plastic Deformation of Directionally-Solidified MoSi 2 /Mo 5 Si 3 Eutectic Composites
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Plastic Deformation of Directionally-Solidified MoSi2/Mo5Si3 Eutectic Composites Yuta Sasai, Atsushi Inoue, Kosuke Fujiwara, Kyosuke Kishida, and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Deformation behavior of the directionally-solidified MoSi2/Mo5Si3 eutectic composites has been investigated as a function of the average thickness of MoSi2 phase over a temperature range from 900 to 1500°C. The average thickness of both MoSi2 and Mo5Si3 phases in the directionally-solidified ingots with script-lamellar morphologies grown by optical floating zone method decreases with increasing the growth rate. Plastic deformation was observed above 1000°C for all the DS ingots grown at different growth rates when the loading axis is parallel to [1¯ 10]MoSi2 close to the growth direction. Yield stress decreases monotonically with increasing temperature. Yield stress at 1400°C increases drastically with decreasing the average thickness of MoSi2 phase. INTRODUCTION In the last decades, there has been increasing demands for new structural materials which can be used in oxidizing environments at higher temperatures than the upper limit for Ni-based superalloys. MoSi2 with the C11b structure has been recognized as one of the most attractive candidates because of its high melting point (2020°C), excellent oxidation resistance, and high thermal conductivity (figure 1a) [1,2]. For the practical application of this material, however, it is essential to improve its low-temperature fracture toughness and high-temperature strength. One possible way is to form an in-situ composite with a secondary phase. Mo5Si3 with the D8m structure (figure 1b) is one of the candidates because it has excellent creep strength at elevated temperatures and also because directionally-solidified MoSi2/Mo5Si3 eutectic alloys possess very high eutectic temperature (1900°C), a fine script-lamellar microstructure composed of a continuous MoSi2 matrix and an interconnected network of Mo5Si3, and better creep properties than the other MoSi2-based composites [3,4]. The orientation relationship between MoSi2 and Mo5Si3 phases in directionally-solidified eutectic compounds has been reported as [1¯10]MoSi2 and [001]Mo5Si3 being approximately parallel with the growth direction, (110)MoSi2 // (110)Mo5Si3 and (001)MoSi2 // (1¯ 10) Mo5Si3 [3]. Mechanical properties of directionally-solidified (DS) eutectic composites are thought to be affected by the thickness of both component phases in the script-lamellar structures which are likely to be controlled by the growth rate in directional solidification. In the present study, we prepared various DS eutectic alloys grown at various growth rate and investigated the effects of the thickness of the script-lamellar structure on the high-temperature strength of the MoSi2/ Mo5Si3 DS eutectic alloys.
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Figure 1. Crystal structures of (a) MoSi2 and (b) Mo5Si3.
EXPERIMENT Ingots of binary MoSi2/Mo5Si3 eutectic alloys with a nominal composition of Mo -54 at.%
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